Transcript Geog 458: Map Sources and Errors

Geog 458:
Map Sources and Errors
January 11, 2006
Data Collection
Outlines
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Overview of data collection methods
Remote sensing
Digitizing
Geographic data formats
1. How do you collect geographic
information?
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Information of geographic features can be captured either
directly or remotely
Go out and survey; mostly it measures distance or direction from
control points; nowadays GPS is increasingly replacing
techniques required for ground surveying
Earth information can be collected without physical contact like
using camera mounted on aircraft or electronic recording
instrument; such a technique is called remote sensing
If some data (directly unusable like it’s in analog form) is already
available, data can be converted into digital data through
scanning or digitizing  secondary data capture
If some data (directly usable but in different file format) is
already available, only file conversion will be necessary  data
transfer
Overview of data input method
Remote Sensing
Primary capture
Data capture
Secondary
capture
Data collection
Data transfer
Data capture vs. data transfer?
Primary data capture vs. secondary data capture?
Ground survey, GPS
Scanning
Digitizing
2. Remote sensing
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Methods of collecting images of the earth remotely
Techniques used to derive information about the physical,
chemical, and biological properties of objects without direct
physical contact
Information is derived from measurements of the amount of
electromagnetic radiation reflected, emitted, or scattered from
objects
Sensors are operated through a variety of electromagnetic
spectrum (from visible to microwave wavelength)
Sensors can be either passive (sun) or active (synthetic aperture
radar: SAR)
Divided into aerial photography and satellite imagery
Group discussions
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Group A: What do three key aspects of resolution
(temporal, spatial, and spectral) refer to?
Group B: What is the difference between air photo and
satellite image?
Group C: What is the difference among air photo,
orthophoto, and digital orthophoto quadrangle (DOQ)?
Group D: Why is knowing about spectral resolution
critical for particular applications?
Three key aspects of resolution
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Temporal: how often (e.g. every 12 days) or repeat cycle
Spatial: pixel size (e.g. 1 meter by 1 meter)
Spectral: in which electromagnetic spectrum
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Single band: sensor is operated in one spectrum
Multispectral: sensor is operated in multiple bands
Hyperspectra: sensor is operated beyond visible bands
Go to EROS data center, and identify three aspects of
resolution in different products
Air photo vs. satellite image
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See Figure 9.2 (p. 203)
Spatial resolution: higher for air photo
Temporal resolution: more variable for air photo
Spectral resolution: more variable for satellite image
Air photo is usually analog; satellite image is intrinsically digital
Air photo is updated in an ad-hoc manner; satellite image is
updated on a regular basis (due to earth-orbiting)
Air photo is good for small area (due to high spatial resolution)
Satellite image is good for large area (due to the availability of
systematic global coverage)
Air photo, orthophoto, DOQs
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Air photo is the photo taken from airplane
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Orthophoto is planimetrically correct air photo
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This does not have a uniform scale, thus you can’t measure
distance correctly using this
Rectification to correct for camera tilt
Conversion from central to parallel perspectives to correct
for radial displacement
DOQs: orthophoto is scanned, georeferenced using
control point, and indexed to quadrangles
Advantage of using active
sensors
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Until very recently, biggest enemy of reading
satellite image was cloud cover
Sensors can be reliant on active light source such
as radar
Thus information can be collected regardless of
weather condition
3. Digitizing
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Manual digitizing: digitizing features on hard
copy map using digitizing equipments
Heads-up digitizing: digitize vector objects
manually straight off a computer screen
Vectorization: the process of converting raster
data into vector data; can be either in batch or
semi-interactive mode; batch program use some
algorithms that extract spaghetti line strings
from the original pixel values
Measurement error
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What is
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Overshoot
Undershoot
Dangling segment
Sliver
See Figure 9.9 to answer
How to deal with it?
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Data cleaning is necessary (for example, GIS software
provides options for snapping, setting tolerance and so on)
4. Geographic data format
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Different geographic data format has evolved to meet
diverse user requirements
CAD file (e.g. DXF, DGN files) is mainly designed to
draw graphics
Raster file format (e.g. jpg, tiff, bil, MrSID) have
developed different compression techniques
Well-known vector format include shapefiles, Arc/Info
coverage, Arc/Info exchange file (*.e00), MIF
(Mapinfo), and VPF file
Conversion between different formats involves
information loss
Spatial Data Transfer Standard
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Aimed at providing robust way of transferring spatial
data between dissimilar computer systems with the
potential for no information loss (from USGS)
Document released in 1992 (revised and approved by
ANSI in 1998) contains standards for self-contained
encoding of spatial data
USGS-related data is provided in SDTS format (e.g.
DLG, DRG, DEM, TIGER/Line)
GIS vendors provide import software
This Friday
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No class
Lab handout is posted in the course website
You can do the lab anytime anywhere
Lab should be done individually
I will have an extended office hour 9:30-12:00 in
my office this Friday to answer any possible
questions
But I would not make it to the class on Friday
(2:30 – 3:50pm)